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| Track Circuits on 3rd Rail vs OHL electrified lines 17/08/2020 at 21:52 #130789 | |
ajax103
1120 posts |
I'm aware of what a track circuit is but I've tried looking at wikipedia to explain more about their use on electrified lines and still am confused. I understand that the track circuits keep the railway separate into different sections of track however it mentions a impedance bond which explains it does the above but also allow DC trains to have a return path for the electricity they draw so is there anyone who can actually explain this better then wikipedia and in terms that I can actually understand please? Also what are the differences for both electrified lines? Log in to reply |
| Track Circuits on 3rd Rail vs OHL electrified lines 17/08/2020 at 22:24 #130792 | |
Steamer
3985 posts 
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As I understand it: Unlike the simple example track circuit you've probably found an explanation of already, most TCs use an AC power supply, which can be at a range of different frequencies. Using arrangements of inductors and capacitors, it's possible to create a 'tuned circuit'. This is a circuit which effectively presents either a high impedance or a low impedance at a specific frequency. Very simplistically, think of an FM radio- altering the setting on the dial changes the specific frequency the radio is allowing to pass (and then amplifying), while cutting out the rest. An impedance bond permits low frequencies- DC (0Hz) or AC traction supply (50Hz)- to pass, while blocking the higher frequency track circuit supplies. A similar method is used on jointless track circuits, where adjacent TCs operate on different frequencies, with tuned circuits at each end used to put the TC supply onto the track and take it off at the other end, while ignoring any other frequencies present. "Don't stress/ relax/ let life roll off your backs./ Except for death and paying taxes/ everything in life.../ is only for now." (Avenue Q) Log in to reply The following users said thank you: ajax103, VInce |
| Track Circuits on 3rd Rail vs OHL electrified lines 17/08/2020 at 22:27 #130793 | |
Albert
1315 posts |
Can't speak for Britain, but in the Netherlands track circuits are used quite extensively with 1500V DC catenary. You can either use one rail only for the return path, or use AC for the track circuits with a choke to block AC in the DC return path. AJP in games Last edited: 17/08/2020 at 22:28 by Albert Reason: None given Log in to reply The following user said thank you: ajax103 |
| Track Circuits on 3rd Rail vs OHL electrified lines 17/08/2020 at 22:31 #130794 | |
GeoffM
6376 posts |
There are many types of track circuit and some are only compatible with certain types of electrification (DC/AC). My former boss was a checker for the TCs around Ashford International when the Eurostart went in. Third rail DC and overhead AC combined with TCs limits the choice somewhat so he had his work cut out for him.
SimSig Boss Log in to reply The following users said thank you: ajax103, 9pN1SEAp |
| Track Circuits on 3rd Rail vs OHL electrified lines 18/08/2020 at 08:57 #130807 | |
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kbarber
1743 posts |
GeoffM in post 130794 said:There are many types of track circuit and some are only compatible with certain types of electrification (DC/AC). My former boss was a checker for the TCs around Ashford International when the Eurostart went in. Third rail DC and overhead AC combined with TCs limits the choice somewhat so he had his work cut out for him.His predecessors who designed the 1960s West Coast Main Line Powerboxes presumably had even more fun, with AC and DC in parallel between Euston & Watford (and using the same lines from Euston to Camden Jc). They didn't have clever electronics in those days, so however they did it they must have used some pretty basic technology. It all worked though. Log in to reply |
| Track Circuits on 3rd Rail vs OHL electrified lines 18/08/2020 at 11:19 #130813 | |
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clive
2789 posts |
kbarber in post 130807 said:From memory they tended to use AC track circuits at frequencies like 33 1/3 and 83 1/3 hertz, as these are least like to appear in leakage from AC or in the "ripples" of rectifier-produced DC. There's also a trick where the feed into one end and what comes out the other end are compared using a transformer or actuator arrangement, so that the circuit knows it's picking up the actual TC feed and not something else. Log in to reply |
| Track Circuits on 3rd Rail vs OHL electrified lines 18/08/2020 at 13:08 #130817 | |
jc92
3689 posts
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kbarber in post 130807 said:GeoffM in post 130794 said:I imagine similar head scratching occurred at Manchester london Road, although thankfully none of the lines shared AC and DC, doubtless there would've been interference.There are many types of track circuit and some are only compatible with certain types of electrification (DC/AC). My former boss was a checker for the TCs around Ashford International when the Eurostart went in. Third rail DC and overhead AC combined with TCs limits the choice somewhat so he had his work cut out for him.His predecessors who designed the 1960s West Coast Main Line Powerboxes presumably had even more fun, with AC and DC in parallel between Euston & Watford (and using the same lines from Euston to Camden Jc). They didn't have clever electronics in those days, so however they did it they must have used some pretty basic technology. When the supertram was built, I understand a number of track circuits at Woodburn and Sheffield had to be replaced to avoid a similar issue. "We don't stop camborne wednesdays" Log in to reply |
| Track Circuits on 3rd Rail vs OHL electrified lines 18/08/2020 at 15:13 #130823 | |
VInce
579 posts |
Steamer in post 130792 said:As I understand it:...and I thought tuned circuits were something only we radio people were familiar with... I thought I'd strayed onto a QRZ.com forum page.. Vince G0ORC I walk around inside the questions of my day, I navigate the inner reaches of my disarray, I pass the altars where fools and thieves hold sway, I wait for night to come and lift this dread away : Jackson Browne - The Night Inside Me Last edited: 18/08/2020 at 15:18 by VInce Reason: None given Log in to reply |
| Track Circuits on 3rd Rail vs OHL electrified lines 18/08/2020 at 16:18 #130830 | |
headshot119
4869 posts |
VInce in post 130823 said:There's quite a bit of overlap it seems between the QRZ forums and here. 2E0CSS "Passengers for New Lane, should be seated in the rear coach of the train " - Opinions are my own and not those of my employer Log in to reply The following user said thank you: VInce |
| Track Circuits on 3rd Rail vs OHL electrified lines 18/08/2020 at 17:50 #130840 | |
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clive
2789 posts |
headshot119 in post 130830 said:VInce in post 130823 said:I sometimes get to deal with tuned circuits that are only microns wide.There's quite a bit of overlap it seems between the QRZ forums and here. Long time since I've used my licence, but I've still got it. G1ZCC Log in to reply The following user said thank you: VInce |
| Track Circuits on 3rd Rail vs OHL electrified lines 18/08/2020 at 18:15 #130841 | |
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VInce
579 posts |
Well well there's more of us about than I thought 73 Vince I walk around inside the questions of my day, I navigate the inner reaches of my disarray, I pass the altars where fools and thieves hold sway, I wait for night to come and lift this dread away : Jackson Browne - The Night Inside Me Log in to reply |
| Track Circuits on 3rd Rail vs OHL electrified lines 19/08/2020 at 10:35 #130855 | |
bugsy
1766 posts
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kbarber in post 130853 said:VInce in post 130841 said:Lots of interesting code stuff.Well well there's more of us about than I thoughtIndeed. Would it have taken those at Bletchley Park long to crack? 
Everything that you make will be useful - providing it's made of chocolate. Log in to reply |
| Track Circuits on 3rd Rail vs OHL electrified lines 19/08/2020 at 11:04 #130857 | |
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ajax103
1120 posts |
GeoffM in post 130794 said:There are many types of track circuit and some are only compatible with certain types of electrification (DC/AC). My former boss was a checker for the TCs around Ashford International when the Eurostart went in. Third rail DC and overhead AC combined with TCs limits the choice somewhat so he had his work cut out for him.Is this why when previously diesel lines are electrified, they have to do the above? Log in to reply |
| Track Circuits on 3rd Rail vs OHL electrified lines 19/08/2020 at 11:06 #130858 | |
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VInce
579 posts |
kbarber in post 130853 said:VInce in post 130841 said:Still active - very active, in fact...Well well there's more of us about than I thoughtIndeed. https://www.qrz.com/db/G0ORC Vince I walk around inside the questions of my day, I navigate the inner reaches of my disarray, I pass the altars where fools and thieves hold sway, I wait for night to come and lift this dread away : Jackson Browne - The Night Inside Me Log in to reply |
| Track Circuits on 3rd Rail vs OHL electrified lines 19/08/2020 at 12:55 #130867 | |
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Steamer
3985 posts
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ajax103 in post 130857 said:GeoffM in post 130794 said:Yes, when a line is electrified the TCs need to be immunised against traction current, or replaced with axle counters.There are many types of track circuit and some are only compatible with certain types of electrification (DC/AC). My former boss was a checker for the TCs around Ashford International when the Eurostart went in. Third rail DC and overhead AC combined with TCs limits the choice somewhat so he had his work cut out for him.Is this why when previously diesel lines are electrified, they have to do the above? We'll not mention that a make of TC branded as traction immune turned out to be susceptible to harmonics commonly generated by electric traction... "Don't stress/ relax/ let life roll off your backs./ Except for death and paying taxes/ everything in life.../ is only for now." (Avenue Q) Log in to reply |
| Track Circuits on 3rd Rail vs OHL electrified lines 19/08/2020 at 13:55 #130868 | |
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jc92
3689 posts
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Steamer in post 130867 said:ajax103 in post 130857 said:Was that related to the issue with (I think) 442s randomly returning signals to danger by spuriously occupying adjacent track circuits?GeoffM in post 130794 said:Yes, when a line is electrified the TCs need to be immunised against traction current, or replaced with axle counters.There are many types of track circuit and some are only compatible with certain types of electrification (DC/AC). My former boss was a checker for the TCs around Ashford International when the Eurostart went in. Third rail DC and overhead AC combined with TCs limits the choice somewhat so he had his work cut out for him.Is this why when previously diesel lines are electrified, they have to do the above? "We don't stop camborne wednesdays" Log in to reply |
| Track Circuits on 3rd Rail vs OHL electrified lines 19/08/2020 at 16:29 #130873 | |
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clive
2789 posts |
I think it was Roger Ford who described cabbing on the Southern somewhere and the driver said "See that green signal? What colour would you like?"
Log in to reply The following user said thank you: VInce |
| Track Circuits on 3rd Rail vs OHL electrified lines 19/08/2020 at 16:57 #130876 | |
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kbarber
1743 posts |
Apparently when the 465s and 466s were introduced the regenerative braking had to be locked out. Permitted for testing in engineers' possession, and a driver told me the signals down beyond the Medway towns looked like Christmas trees when they used it, flashing effects and all!
Log in to reply The following user said thank you: jc92 |
| Track Circuits on 3rd Rail vs OHL electrified lines 19/08/2020 at 18:21 #130879 | |
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GeoffM
6376 posts |
kbarber in post 130807 said:GeoffM in post 130794 said:Maybe things were simpler back then, much the same way my dad was a wizard with the BT fleet (vans, trucks, piledrivers, all sorts of mechanical equipment) when it was all mechanical but became more and more confuddled with computerisation. Thankfully he was near retirement by that point anyway.There are many types of track circuit and some are only compatible with certain types of electrification (DC/AC). My former boss was a checker for the TCs around Ashford International when the Eurostart went in. Third rail DC and overhead AC combined with TCs limits the choice somewhat so he had his work cut out for him.His predecessors who designed the 1960s West Coast Main Line Powerboxes presumably had even more fun, with AC and DC in parallel between Euston & Watford (and using the same lines from Euston to Camden Jc). They didn't have clever electronics in those days, so however they did it they must have used some pretty basic technology. A couple more things I remember my boss saying. Firstly, preservation (steam) railways often get some free equipment in return for using their track: the West Somerset railway got some great track beds from tamper training; and the Dart Valley got free track circuiting. He was probably in his element using steam locos to test his track circuits! He also said the TCs around Ashford had to be carefully laid out as certain frequencies can't be near other frequencies. Get one frequency wrong and you now have to change all the TCs around it - and you have a limited number of frequencies you can use. Way over my head so some of my terminology may be incorrect. SimSig Boss Log in to reply The following user said thank you: VInce |
| Track Circuits on 3rd Rail vs OHL electrified lines 19/08/2020 at 19:07 #130880 | |
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Stephen Fulcher
2080 posts |
It's worth noting that it's not just electrified lines that have to be considered. A lot of the Chiltern line between Marylebone and West Ruislip uses reed track circuits rather than the normal DC due to its proximity to the Underground. Axle counters aren't entirely immune to interference from traction current either. IETs had an influence over a certain type of AzLM on the western which could cause them to fail, although I'm not sure of the specifics. Log in to reply The following user said thank you: VInce |
| Track Circuits on 3rd Rail vs OHL electrified lines 20/08/2020 at 10:30 #130892 | |
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Chromatix
190 posts |
The newer electric traction types with switching inverters doubtless impose a lot more harmonics on the supply (and return path) than the old tap-changing and resistance-bank machinery. This is evident at least from the space-invader noises they make when moving off - that's the harmonics acting to vibrate cables and other components on the train! The manufacturer does have some influence over the frequencies produced by these inverters. Siemens used to have a model which was tuned to produce a musical scale, famously used on the Keikyu line in Tokyo. Probably some of the back and forth in traction compatibility is in figuring out which traction frequencies cause trouble with the S&T stuff, and negotiating who needs to change what. Log in to reply |
| Track Circuits on 3rd Rail vs OHL electrified lines 22/08/2020 at 17:57 #130956 | |
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markt
60 posts |
kbarber in post 130853 said:VInce in post 130841 said:I feel lost around all these bigger call signs...Well well there's more of us about than I thoughtIndeed. Newly renewed M3XHZ. The call sign seems appropriate for track circuit frequencies. 
20 years of Simsigging Log in to reply The following user said thank you: VInce |